Hundreds of millions of aluminum die-cast parts are made in the United States every year. These parts are produced by many manufacturers on a number of different types of casting machines, but quality and yield of the die-cast parts are concerns common to all manufacturers. For those areas of the market where quality is a primary concern, particularly for suppliers to the automotive industry, new improved die-casting processes need to be improved to the extent that die-cast pieces can replace aluminum forgings.
Molten metal transfer for aluminum die casting and other processes generally includes two operations: transfer of the molten metal from the melting furnace to the holding furnace, and transfer from the holding furnace to the shot chamber of the die-casting machine. Usually, a single large-volume melting furnace is used to supply molten metal to smaller holding furnaces at die casting machines. The transfer to the die casting machine is usually accomplished by manual or mechanized ladling. Some problems associated with transfer ladles are the buildup of dross, and agitation, leading to non-metallic inclusions in the product, and the labor intensive nature of such transfer. Electromagnetic pumps have been used to some extent in transfer to the die-casting machine, but those available have proven to be rather fragile and somewhat costly to maintain.
U.S. Pat. No. 4,212,592 (Olich et al.), teaches a self-priming, low friction, electromagnetic, partial-immersion pump for molten metals. There, a rotating magnetic field, perpendicular to molten aluminum flow, generates currents in the molten metal, and with an inlet swirl inducer causes rotation of the molten metal about the axis of the pump structure, which comprises a high permeability stainless steel cylinder covered with ceramic mat and silicon carbide. The currents produced, interact with the magnetic field and a torpedo inductor to provide a forward propelling force in the molten metal. The swirl inducer of the torpedo comprises a plurality of spaced apart openings along the sides of the torpedo inductor. In this patent an annular ring extends from and circumscribes the solid torpedo inductor body.
In U.S. Pat. No. 4,842,170 (Del Vecchio et al.), monophase electromagnetic field operations are detailed for flow control devices used to control molten metal flow. Here, a variety of torpedo styles are utilized. One torpedo style includes a solid central cylindrical post and a plurality of circumferentially spaced apart vanes extending radially outward from the post. Another style is a cylindrical plug sized to fit closely within a pump nozzle orifice, where the plug has either a plurality of transversely spaced apart bores (12 are shown) extending axially through the plug, or where the plug is an annular ring having a plurality of circumferentially spaced apart interior ribs to define a large "multi-petal shaped" interior space. Similar type torpedo styles are described in U.S. Ser. No. 07/945,456 filed on Sept. 16, 1992 (Slepian et al. W.E. Case No. 57,142). In all of these torpedo styles, no mechanism is provided for reverse flow. The magnetic forces would tend to drive the molten metal in the pumping direction.
The Del Vecchio et al. and Slepian et al. inventions describe how an axial force can be developed in a molten metal column through the interaction of high frequency electromagnetic fields and induced eddy currents. A device generating this force can be used as either a pump or valve. In developing the force, heat is inherently generated within the molten metal stream. In addition to thermal and mechanical stresses that may develop in the pump as a result of the molten metal overheating, the temperature of the metal delivered to the caster may easily exceed the margins permitted.
Die casting machines work on a cyclical basis in which a controlled volume of metal within a cylinder is periodically injected into the mold of the machine by a ram. It would be desirable to hold the metal close to the cylinder between shots for several reasons. Exposure of the molten aluminum to air results in oxidation of the aluminum, forming inclusions in the casting and thereby reducing the quality of the part. If the aluminum is held in a small diameter pipe near the cylinder, contact with the air is minimized. The other reason is one of economics, the closer the metal is to the cylinder, the faster it can be filled. If the aluminum is allowed to drain from the transfer pipe back into the furnace each time, additional cycle time will be required to pump the aluminum from the furnace to the caster.
What is needed is a means to allow use of electromagnetic fields to operate a molten metal pump in a holding application, where a prolonged static head is maintained while avoiding excessive heating of the molten metal in the pump. It is one of the main objects of this invention to provide such a means.